The present invention relates to an OCT optical tomographic imaging system, tomographic image acquiring method, and optical tomographic image forming method that employ a wavelength-swept light source.
Acquisition of a cross-sectional image of a measuring object, i.e., an object to be measured, such as biological tissue without cutting thereinto may be achieved using an optical tomographic imaging system employing optical coherence tomography (OCT) measuring.
The OCT measuring is a kind of optical interferometric measurement using the optical interference that occurs only when the optical path lengths of the measuring light and the reference light, into which the light from the light source is divided, are matched to within the coherence length of the light from the light source.
Known optical tomographic imaging systems include systems that are based on Time Domain OCT (TD-OCT), which obtain a tomographic image by changing the optical path length of the reference light so as to vary the measuring position (measured depth) of the measuring object, and systems based on Spectral Domain OCT (SD-OCT) and Swept Source OCT (SS-OCT), which obtain a tomographic image in the optical axial direction by measuring the intensity of the interference light of each frequency component so as to obtain the Fourier transform of the spectral interference waveform, rather than by changing the optical path length of the reference light.
SS-OCT employing a wavelength-swept light source adjusts the reference position (zero path position) where the optical path lengths of a signal light (measuring light) from a target object and reference light match so that the reference position falls within a desired measurement range within the coherence length of the light source and, once the zero path position is adjusted, fixes the zero path position to that position. With this arrangement, the interference signal is then detected, the Fourier transform of the detected interference signal is obtained, and the absolute value of each characteristic best frequency is plotted, thereby acquiring a tomographic image without changing the optical path length.
In particular, in the SS method and TD method, the measurement range that can be acquired at the same time is limited to within the coherence length of the light source. To this end, for example, JP 2003-516531 A, addresses the OCT system used in a funduscopy apparatus and the difficulties that arise in interpretation in comparison to a conventional Scanning Laser Ophthalmoscope (SLO) due to the short coherence length and the resultant OCT horizontal images that show only segments of the retina, and proposes an optical mapping system that creates a horizontal image by employing an OCT that uses a light source having an extremely short coherence length of a range from 10 μm to 300 μm, changing a reference optical path in stages so as to collect horizontal OCT images of different depths, and then processing the collected horizontal OCT images with software.
This technology attempts to obtain an OCT image across an immeasurable range based on a single measurement by changing the reference optical path in stages and collecting OCT images.